42 BRITISH FOREST TREES 



Increase of growth in girth is of course to a great degree 

 dependent on the situation and the quality of the soil, but 

 factors of almost equal importance are the degree of light, 

 and the measure to which the available light can be utilised, 

 or in other words, to the extent of the development of 

 foliage. In crowded forests, growth in girth is injuriously 

 affected by diminution of the supply of light even more 

 than growth in height ; in according growing-space to the 

 individual beyond a certain degree, however, growth in girth 

 takes place somewhat at the expense of growth in length. 

 When trees in the full vigour of growth are given an in- 

 crease of growing-space after having been accustomed only to 

 a limited space, the influence of the larger measure of enjoy- 

 ment of light and air becomes marked in the rapid increase 

 in girth. Even when the trees are advanced in age, and 

 have already begun to decrease in the rate of girth incre- 

 ment as for example in the case of beech or silver fir, 

 during gradual clearances at the time of natural reproduction 

 from seed under standards a revival of vigorous increment 

 can be induced by giving them free access to light and air. 

 Such ability is, however, dependent on the trees not yet 

 having completed their growth in height, as the energy 

 employed in extending the foliage-bearing crown is really 

 diverted from growth in height and utilised in branch 

 development. Silver fir, spruce, larch, oak, and beech, 

 endowed with long-continued capacity for growth in height, 

 also retain longest the power of benefiting by increased 

 supplies of light and air. 



The largest girths are attainable by oak, elm, chestnut, 

 lime, beech and black poplar among broad-leaved trees, and 

 silver fir, spruce, Weymouth and Scots pines, Douglas 

 fir, and larch among conifers. 



(c) Growth in Cubic Contents, or Total Increment, is the 

 complete result attained by the sums total of growth in height 



